This invention relates to an electric circuit for lighting a lamp or lamps such as, typically, metallic halide lamps or like discharge lamps used on motor vehicles. More particularly, the invention deals with such a lamp lighting circuit featuring provisions for automatically disconnecting the lamp from the power supply in the event of abnormal circuit conditions such as caused by a lamp failure or an excessive supply voltage.
Incandescent lamps such as halogen-cycle lamps have long been the standard light sources of automotive headlamp assemblies. The known lighting circuit for such incandescent headlamps has been of very simple design, with the lamps connected to a battery via relay contacts and with a lamp switch connected to the battery via the relay coil. No means for circuit protection from overload have usually been incorporated with the lighting circuit.
A variety of troubles unavoidably develop in the vehicular lighting circuit. Such troubles include, for example, those brought about at or toward the end of the service life of the light bulbs, and battery overvoltage. Or the battery may be connected to the lighting circuit in the wrong polarity. The worst possible outcome of these troubles in the conventional lighting circuit for incandescent headlamps has been the breakage of the lamp filaments. Overload protection means have therefore been not an absolute necessity.
Recent years, however, have witnessed the advent of metallic halide lamps as the light sources of vehicle headlamp assemblies in substitution for incandescent lamps as typified by halogen lamps. This new type of discharge lamp requires a starting voltage of as high as 10 to 20 kilovolts in order to be instantly lit up for proper functioning as vehicle headlamps. Such high voltage can be a cause of serious consequences without overvoltage protection.
Should the lamp fail to glow for its own trouble upon actuation of the lamp switch, the high voltage would continue developing between the output terminals of the lighting circuit in the absence of overload protection means. The unconsumed high voltage might disrupt the insulations between the contacts within the lamp sockets. The possible result might be a fire. Or the user might be struck by the high voltage in an inadvertent attempt to replace the lamps. Battery overvoltage, and battery connection in the wrong polarity, might also ruin the circuit elements, possibly resulting in the development of dangerously high output voltage and in the eventual destruction of the lamps.
The lighting circuit for metallic halide lamps has had additional causes of trouble in connection with the d.c. voltage booster circuit incorporated therein for controllably elevating the battery voltage prior to conversion into a.c. voltage required by the metallic halide lamps. The booster circuit is prone to develop overvoltage or overcurrent in response to various abnormal circuit conditions. The lighting circuit and the lamps must also be protected against such booster overvoltage and overcurrent.